Combined effects of the inlet temperature and the wall thermal capacitance on the laminar mixed convection in a vertical pipe

Abstract

PurposeThe aim of this study is to present numerical analyses for combined effects of the inlet temperature (ΔT+) and the wall‐to‐fluid thermal capacitance ratio (a*) on the laminar mixed convection unsteady flows in a vertical pipe.Design/methodology/approachThe full Navier‐Stokes and energy, coupled, unsteady state, two‐dimensional governing equations for ascending laminar mixed convection in a vertical pipe are solved numerically using a finite‐difference scheme.FindingsThe results show that the thermohydraulic flow behaviour is highly dependent on both parameters (ΔT+, a*). Moreover, the unsteady characteristics of the flow can involve oscillatory and reversed flow phenomena yielding the unstable flows. For the heating case, the reversed flow appears below the wave instability and the unsteady vortex is always significant in the vicinity of the wall, whatever ΔT+ and a*<100. For the cooling case, the reversed flow appears in the central region of the pipe; it develops on top of the wave instability.Practical implicationsThis study should be very useful to improve heat transfer equipment.Originality/valueThe paper shows clearly the combined effects of both parameters (ΔT+, a*) on the laminar mixed convection flow.